Standard Heterogeneous Rate Constant Research Articles

Gold electrodes modified with 16H,18H-Dibenzo[c,l]-7,9-dithia-16,18-diazapentacene for electrocatalytic oxidation of NADH

16H,18H-Dibenzo[c,l]-7,9-dithia-16,18-diazapentacene (DDDP), a new phenothiazine derivative containing two linearly condensed phenothiazine rings, strongly adsorbs on polyoriented gold resulting in a modified electrode with electrocatalytic activity for β-nicotinamide adenine dinucleotide (NADH) oxidation. Cyclic voltammetry (CV) measurements, performed in aqueous buffer solutions at different pH values, allowed to: (i) estimate the standard heterogeneous rate constants ( k S ≈1 s −1 , at pH 7.0) in the pH range between 4 and 8; (ii) demonstrate the electrocatalytic activity of the investigated compound for NADH oxidation. The second-order electrocatalytic rate constant k 1 was evaluated from the rotating disk electrode (RDE) technique ( k 1,[ NADH]=0 =5.0×10 3 M −1 s −1 , at pH 7.0). The electrochemical behavior and electrocatalytic activity of DDDP are markedly influenced by the intramolecular interaction between the two conjugate phenothiazine rings. The electrocatalytic activity of DDDP (1e −, 1H + acceptor) was assigned to its ability to extract rapidly from NADH the first electron and the proton.

The electrochemically induced conformational transition of disulfides in bovine serum albumin studied by thin layer circular dichroism spectroelectrochemistry

The conformational transition of disulfides in bovine serum albumin (BSA) induced by electrochemical redox reaction of disulfides were monitored by in-situ circular dichroism (CD) spectroelectrochemistry, with a long optical path thin layer cell and analyzed by a singular value decomposition least square (SVDLS) method. Electrochemical reduction of disulfides drives the left-handed conformation of disulfides changed into the right-handed. At open circuit, eight of the 17 disulfides were of left-handed conformation. Four of the 17 disulfides took part in the electrochemical reduction with an EC mechanism. Only one-fourth of the reduced disulfides returned to left-handed conformation in the re-oxidation process. Some parameters of the electrochemical reduction process, i.e. the number of electrons transferred and electron transfer coefficient, n=8, α n=0.15, apparent formal potential, E 1 0′=−0.65(±0.01) V, standard heterogeneous electron transfer rate constant, k 1 0=(2.84±0.14)×10 −5 cm s −1 and chemical reaction equilibrium constant, K c=(5.13±0.12)×10 −2, were also obtained by double logarithmic analysis based on the near-UV absorption spectra with applied potentials.

Solid deposits of osmium bis-bipyridyl triazole chloride: Redox properties and electrocrystallisation

Mechanically attached, solid-state films of [Os(bpy)2 bpt Cl] have been formed on platinum microelectrodes and their voltammetric properties investigated, bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4-triazole. Scanning electron microscopy reveals that voltammetric cycling in 1.0 M HClO4 converts the amorphous array of microscopically small particles into a plate-like semi-crystalline form. In contrast, crystallisation does not occur when the films are cycled in 1.0 M NaClO4. In both electrolytes, the voltammetric response of these films is reminiscent of that observed for an ideal reversible, solution phase redox couple. Slow and fast scan linear sweep voltammograms have been used to provide an absolute determination of the fixed site concentration and apparent diffusion coefficient, Dapp. The fixed site concentration is 1.65 ± 0.05 M for films cycled in either electrolyte and the Dapp values increase with increasing electrolyte concentration, Celec. These observations suggest that ion transport rather than the rate of electron self-exchange limit the overall rate of charge transport through these solids. In 1.0 M NaClO4, Dapp values for oxidation and reduction are identical at 8.3 ± 0.5 × 10−12 cm2 s−1. In 1.0 M HClO4, Dapp is significantly lower and depends on whether the deposit is being oxidised (9.7 ± 0.4 × 10−13 cm2 s−1) or reduced (6.3 ± 0.4 × 10−13 cm2 s−1). These data have been used to obtain an insight into the relative importance of intra- s. inter-particle charge transport. When Celec>0.5 M, the standard heterogeneous electron transfer rate constant, k°, becomes independent of the electrolyte concentration with a value of 1.7 ± 0.2 × 10−5 cm s−1 being observed in both 1.0 M NaClO4 and HClO4. Significantly, the distance normalised heterogeneous electron transfer rate constant for these solid state films is almost three orders of magnitude smaller than that found within a spontaneously adsorbed monolayer of the same complex. The importance of these results for the rational design of solid-state redox active materials for battery, display and sensor applications is considered.

Effect of Electrode Density of States on the Heterogeneous Electron-Transfer Dynamics of Osmium-Containing Monolayers

Dense monolayers of [Os(OMe-bpy)2(p3p)Cl]1+, where OMe-bpy is 4,4‘-dimethoxy-2,2‘-bipyridyl and p3p is 4,4‘-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum, mercury, gold, silver, carbon, and copper microelectrodes. These systems have been used to probe the influence of the electrode density of states on the rate of electron transfer across the electrode/monolayer interface. Monolayers on each material exhibit well-defined voltammetry for the Os2+/3+ redox reaction where the supporting electrolyte is aqueous 1.0 M NaClO4. The high scan rate (>2000 V s-1) voltammetric response has been modeled using a nonadiabatic electron-transfer model. The standard heterogeneous electron-transfer rate constant, k°, depends on the identity of the electrode material, e.g., k° is 6 × 104 and 4 × 103 s-1 for platinum and carbon electrodes, respectively. Chronoamperometry, conducted on a microsecond time scale, has been used to probe the potential dependence of the heterogeneous electro...

Conformational transition of DNA in electroreduction studied by in situ UV and CD thin layer spectroelectrochemistry

Electrochemically induced three conformational transitions of calf thymus DNA from B 10.4 to Z 10.2-DNA and from B 10.2 to B 10.4 and to C-DNA in 10 mM phosphate buffer solution (pH 7.21) at glassy carbon electrode are found and studied by in situ circular dichroism (CD) thin layer spectroelectrochemistry with singular value decomposition least square (SVDLS) analysis. It indicates that the so-called B 10.2 form and the C-form of DNA may be composed of B 10.4 and left-A DNA and of B 10.4 and right-A DNA, respectively. The irreversible electrochemical reduction of adenine and cytosine groups in the DNA molecule is studied by UV-Vis spectroelectrochemistry. Some electrochemical parameters α n=0.17, E 0′=−0.70 V (vs. Ag/AgCl), and the standard heterogeneous electron transfer rate constant, k 0=1.8×10 −5 cm s −1, are obtained by double logarithmic analysis and non-linear regression.

In situ Monitoring of Lysozyme Adsorption onto Bare and Cysteine- or 1-Octadecanethiol-Modified Au Electrodes Using an Electrochemical Quartz-Crystal Impedance System

The adsorption of lysozyme from phosphate-buffered saline (pH 7.4) onto bare and cysteine- or 1-octadecanethiol-modified Au electrodes was investigated in situ using an electrochemical quartz-crystal impedance system (EQCIS). The equivalent circuit parameters and resonant frequencies of the piezoelectric quartz-crystal (PQC) resonance, the value of half peak width of the electroacoustic conductance spectrum (DfG1/2) as well as the electrical double-layer capacitance were obtained and analyzed. The adsorption kinetics was analyzed with a scheme of two consecutive reactions occurring at the interface. The amount of adsorbed lysozyme on a hydrophobic surface was greater than that on a hydrophilic one, and a more negative potential brought a larger amount of adsorbed lysozyme. The standard heterogeneous rate constants of ferricyanide/ferrocyanide before and after the lysozyme adsorption were evaluated.

Computer-controlled instrumentation for fast voltammetry at ultramicroelectrodes

In this article we describe an electrochemical experimental setup for cyclic voltammetry operating from low (1 V/s) to very high (200 kV/s) scan rates. The system is designed to achieve an acceptable cost and to be user friendly for nonexperts. The instrumentation is based on a commercial arbitrary wave form generator, a digital oscilloscope, a general purpose interface bus interface to a PC, and a laboratory-built potentiostat. There are no complicated manual operations, instead the software creates dialog with an operator to control commands for the instruments. Requirements for the wave form synthesis and the data acquisition are given. Various data-smoothing methods for treating 8-bit fast scan data are evaluated. The performance was tested on the oxidation of ferrocene in acetonitrile. The estimated value of the standard heterogeneous rate constant, k0=0.8±0.1 cm s−1, is in reasonable agreement with the most recent reports from other laboratories.

Electrochemical studies of heterogeneous reduction of tetracyanoquinodimethane in poly(ethylene oxide) electrolytes using ac impedance and cyclic voltammetry at an ultramicroelectrode

Electrochemical studies of the TCNQ0/TCNQ− couple have been carried out using ac impedance spectroscopy and cyclic voltammetry at platinum ultramicroelectrodes (UME). Liquid poly(ethylene oxide) (PEO) CH3–O–(CH2–CH2–O)4–CH3 has been used as the solvent with different concentrations of the TCNQ0/TCNQ− couple and LiClO4 as the supporting electrolyte. On the basis of the ac impedance results at the UME it has been found that the double layer capacitance and standard heterogeneous rate constant are independent of the presence of electroactive species and supporting electrolyte concentrations, indicating that adsorption of electroactive species onto the electrode is not significant. The standard heterogeneous rate constant ks was found to be 0.109 ± 0.005 cm s−1. A similar value of ks = 0.094 cm s−1 was obtained for the second reduction step TCNQ−/TCNQ2−. Diffusion coefficients of both TCNQ and TCNQ− are equal, D = 1.0 × 10−6 cm2 s−1 for a 0.5 M LiClO4 solution. Higher diffusion coefficients are obtained in less concentrated supporting electrolyte. Comparison is made between these results and those reported previously for PEO-400 HO–(CH3CH2O)8–OH. The different end groups significantly influence the viscosity and hence ks.

Evaluation of uncompensated solution resistance for electrodes with spherical-cap geometry

Typical custom and commercial hanging mercury drop electrodes have the geometry of a spherical cap formed by the plane of the lower surface of the electrode holder cutting off the top of the drop. To conduct accurate quantitative measurements by voltammetry, it is necessary to be able to account for the effects of solution resistance, Ru. A method of determining the solution resistance is proposed and tested. The method involves making measurements of a test reaction (in this case, oxidation of ferrocene) by cyclic voltammetry at scan rates where resistance effects are important and at more than one concentration. When the data are analyzed by digital simulation, it is found that only one value of Ru will provide adequate matches between simulation and experiment at all concentrations. An approximate equation has been derived that allows calculation of Ru from the dimensions of the spherical-cap electrode and the solution resistivity. The calculated values of Ru for electrodes of three different sizes agreed well with the measured values. Error analysis was performed for a particular measurement, the determination of the standard heterogeneous electron-transfer rate constant, ks, by cyclic voltammetry, and it was shown that uncertainty in Ru puts an upper limit of about 1 cm/s for the determination of ks with the hanging mercury drop electrodes used in this study.

Combined quartz crystal impedance and electrochemical impedance measurements during adsorption of bovine serum albumin onto bare and cysteine- or thiophenol-modified gold electrodes

Combined piezoelectric quartz crystal (PQC) impedance and electrochemical impedance measurements as a novel technique were reported for monitoring adsorption of bovine serum albumin (BSA) onto bare and cysteine- or thiophenol-modified gold electrodes. Equivalent circuit parameters and resonant frequencies for PQC resonance as well as the electrical double layer capacitance were obtained and discussed. A scheme of two consecutive reactions occurring at the interface was used to analyze the adsorption kinetics, showing that the double layer capacitance is more sensitive to the re-arrangement of the adsorbed protein molecules compared with the resonant frequency. The standard heterogeneous rate constants of ferricyanide/ferrocyanide before and after the BSA adsorption were determined. Since the present technique provides not only electrochemical impedance information but also multidimensional piezoelectric information simultaneously, it is highly recommended for wider applications in electrochemical studies.

Electrochemistry of Gallium in the Lewis Acidic Aluminum Chloride‐1‐Methyl‐3‐ethylimidazolium Chloride Room‐Temperature Molten Salt

The electrochemistry of gallium at tungsten and glassy carbon was investigated in the 60.0/40.0 mol % aluminum chloride‐1‐methyl‐3‐ethylimidazolium chloride molten salt. The electrodissolution of gallium produces a gallium(I) solution, which can be electrochemically oxidized to Ga(III). The formal potential for the Ga(III)/Ga(I) and Ga(I)/Ga(0) couples are 0.655 and 0.437 V, respectively. The Ga(I)/Ga(III) electrode reaction exhibits slow‐charge transfer kinetics with an anodic transfer coefficient and a standard heterogeneous rate constant of 0.24 and , respectively, at tungsten, and 0.14 and , respectively, at glassy carbon. The electrodeposition of gallium at glassy carbon from solutions of Ga(I) exhibits instantaneous three‐dimensional nucleation with diffusion‐controlled growth of the nuclei. The Ga(III) species can react with gallium metal to form Ga(I). The diffusion coefficients for Ga(III) and Ga(I) species are and , respectively. © 1999 The Electrochemical Society. All rights reserved.

Redox Properties of Ground and Electronically Excited States: [Ru(bpy)2Qbpy]2+ Monolayers

Dense monolayers of [Ru(bpy)2Qbpy]2+, where bpy is 2,2‘-bipyridyl and Qbpy is 2,2‘:4,4‘‘:4‘4‘‘-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and five redox states are accessible over the potential range from +1.3 to −2.0 V. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions. Heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s-1). Standard heterogeneous electron-transfer rate constants, k°, have been evaluated by extrapolating Tafel plots of ln k vs overpotential, η, to zero driving force to yield values of (5.1 ± 0.3) × 105 s-1, (3.0 ± 0.1) × 106 s-1, and (3.4 ± 0.2) × 106 s-1 for k°3+/2+, k°2+/1+, and k°1+/0, respectively. Temperature-resolved measurements of k reveal that the electrochemical activation enthalpy, ΔH⧧, decreases f...

Electrochemistry of Thallium in the Basic Aluminum Chloride‐1‐methyl‐3‐ethylimidazolium Chloride Room Temperature Molten Salt

The electrochemistry of thallium was studied in the basic aluminum chloride‐1‐methyl‐3‐ethylimidazolium chloride molten salt at 30°C. In this basic melt system, thallium (III) chloride is complexed as and can be electrochemically reduced to a Tl(I) species. The Tl(III)/(I) electrode reaction exhibits slow charge‐transfer kinetics with a standard heterogeneous rate constant of and a cathodic transfer coefficient of ca. 0.2 in the 44.4/55.6 mole percent (mol %) melt. The formal potential of this redox couple is dependent upon the chloride concentration of the basic melt, suggesting the loss of two chloride ions from coordination sphere during reduction to produce ) which could be further reduced to thallium metal. The formal potentials of the Tl(I)/Tl couple in the 44.4/55.6 and 40.0/60.0 mol % melts are −0.965 and −0.900 V, respectively, vs. Al(III)/Al in the 66.7/33.3 mol % melt. The electrodeposition of thallium on both tungsten and nickel electrodes proceeds via instantaneous three‐dimensional nucleation.

Electrochemical Studies of Chromium(III) and Chromium(II) Chloride Complexes in Basic Aluminum Chloride‐1‐Methyl‐3‐ethylimidazolium Chloride Room Temperature Molten Salts

Chromium(III) chloride is complexed as in the basic aluminum chloride‐1‐methyl‐3‐ethylimidazolium chloride melt, and it can be reduced to a Cr(II) species at a glassy carbon electrode. Absorption spectroscopy and titration experiments suggest that the Cr(II) is complexed as . The Cr(III)/Cr(II) electrode reaction exhibits slow charge‐transfer kinetics with a standard heterogeneous rate constant of and a cathodic transfer coefficient of ca. 0.38 in the 44.4/55.6 mole percent (m/o) melt at 27°C. The formal electrode potential of the Cr(III)/Cr(II) chloride redox couple is −0.649 V in the 44.4/55.6 m/o melt. The Stokes‐Einstein products of and complexes are and respectively.

Electrochemical Study of the Gold Thiosulfate Reduction

The electrochemical reduction of gold thiosulfate has been studied and compared to the reduction of gold cyanide. Gold thiosulfate is a potential replacement for gold cyanide in electro and electroless plating baths. Gold thiosulfate has a more positive reduction potential than gold cyanide and eliminates the use of cyanide. The standard heterogeneous rate constant, transfer coefficient, and diffusion coefficient for gold thiosulfate reduction were found to be , 0.23 and , respectively. The effect of sulfite as an additive to gold thiosulfate solutions was examined.

Electron-transfer reactions accompanied by substantial structural changes: oxidation of 9,10-bis(dimethylamino)anthracene and 3,6-bis(dimethylamino)durene

The electrochemical oxidation of N , N , N ′, N ′-tetramethyl-1,4-phenylenediamine ( 1 ), 9,10-bis(dimethylamino)anthracene( 2 ) and 3,6-bis(dimethylamino)durene ( 3 ) has been studied at glassy carbon electrodes in acetonitrile. 1 shows two one-electron oxidation steps with the normal ordering of potentials, the second electron being 0.6 V more difficult to remove than the first. Each electron-transfer reaction is inherently rapid. 2 and 3 , by contrast, show a single two-electron oxidation peak by cyclic voltammetry. Potentiometric titrations suggest for 2 and demonstrate for 3 that the reversible formal potentials are inverted for these species, with the second electron being removed more easily than the first. For 3 , the overall two-electron reversible potential is −0.035 V vs. ferrocene, and the difference in the one-electron formal potentials is estimated to be more than 0.20 V. Evaluation of the standard heterogeneous electron-transfer rate constants for the individual oxidation steps was carried out for 2 and 3 , and it was found that the values are unusually small, much smaller than seen for 1 . This observation has been interpreted to mean that substantial structural change accompanies the oxidation of 2 and 3 , but not 1 . AM1 calculations have been used to illuminate the causes of sluggish electron-transfer kinetics for 2 and 3 , and to rationalize the observed inversion of the potentials.

An ultramicroelectrode study of ferrocene diffusion and heterogeneous electron transfer rate in polyelectrolyte

Steady-state voltammograms at a microdisk electrode are used to measure the diffusion coefficient ( D) and standard heterogeneous rate constant ( k s ) of ferrocene in polyelectrolyte PEG · MClO 4. The diffusion coefficient and standard heterogeneous rate constant of ferrocene are both smaller in polymer solvents than in monomeric solvents. The D and k s of ferrocene have been estimated in PEG containing different concentrations and cations of supporting electrolytes, and the dependencies of D and k ms on temperature have been observed. These results show that the D and k s of ferrocene increase with increasing temperature in polyelectrolyte, and with increasing cation radius of supporting electrolyte, eg D and k s increase in the order Bu 4NClCO 4 > NaClO 4 > LiClO 4. On the other hand, D and k s increase with decreasing concentration of supporting electrolyte. The dependence of the half-wave potential ( E 1 2 ) on the concentration of the supporting electrolyte is also observed. E 1 2 shifts in the negative direction as the concentration of supporting electrolyte increases.

Determination of the Charge Transfer Parameters of 4,4'-(Ferrocene-1,1'-diyldivinylene)bis(benzo-15-crown-5) at Glassy Carbon Electrode

Electrochemical parameters of 4,4'-(ferrocene-1,1'-diyldivinylene)bis(benzo-15-crown-5) were collected using cyclic voltammetry, chronoamperometry and chronopotentiometry at a glassy carbon electrode. One-electron quasi-reversible oxidation wave was obtained at Epa of 0.532 V and Epc of 0.387 V. The peak separation, ∆Ep, standard heterogeneous rate constant, k0, and diffusion coefficient, D, of the electrochemical species have been processed by convolution and deconvolution transformations. Digital simulations using the kinetic parameters were also performed.

Measurement of Heterogeneous Electron Transfer Rate Constants for Fe(H2O)62+/3+during Sonication of Suspensions of Alumina Particles

Heterogeneous electron transfer kinetics for reduction of Fe(H2O)63+ at Pt electrodes in the presence of equimolar Fe(H2O)62+ in noncoordinating 1.0 M HClO4 have been studied during irradiation with high-intensity ultrasound at 20 kHz both in the presence and in the absence of small solid particles. Increases in heterogeneous rate constants have been measured during sonication in the presence of 1-μm particles of Al2O3. During sonication, particles collide with the electrode surface, generating heat and enhancing kinetic rates relative to those determined for sonication in the absence of particles. Control experiments (in the absence of ultrasound) were conducted with a rotating disk electrode (RDE). Rate constants from charge transfer resistance measurements within ±0.025 V of the equilibrium potential obtained by RDE, sonication, and sonication with 16.7 g of Al2O3 per liter of solution were (3.4 ± 0.1) × 10-4, (4.2 ± 0.5) × 10-4, and (20 ± 2) × 10-4 cm/s, respectively. By comparison, Tafel plots corrected for mass transport effects gave rate constants (RDE, sonication, and sonication with Al2O3) of (4.4 ± 0.2) × 10-4, (6 ± 1) × 10-4, and (15 ± 5) × 10-4 cm/s, respectively. Additional studies indicated that no increases in electrode area or rate constants were measurable postsonication; thus, rate enhancements are attributed to thermal effects (cavitation and collisions). Similar enhancements were also found for Fe(CN)63- reduction in the presence of equimolar Fe(CN)64-. Nonsonication, variable-temperature studies of Fe(H2O)62+/3+ gave activation energies for the kinematic viscosity, Fe(H2O)63+ diffusion coefficient, and standard heterogeneous rate constant of 15.8, 14.9, and 12.1 kJ/mol, respectively. From the Arrhenius relationship, effective temperatures at the electrode surface during sonication of a solution held at 273 K were found to be (282 ± 5) K without Al2O3 and (410 ± 10) K with 16.7 g of Al2O3 per liter of solution. The addition of larger quantities of Al2O3 produced even greater effects.

Migrational chronoamperometry of uncharged substrates. influence of electron transfer rate

An improved numerical way of solving the second Fick's law for the purpose of modelling chronoamperometry of uncharged species in a solution containing an insufficient amount of supporting electrolyte is presented. The simulation of the mixed diffusional and migrational transport is based on the Crank-Nicolson method with an exponentially expanded space grid. The simulation scheme includes the electron transfer rate and assumes the use of hemispherical microelectrodes. The electroneutrality condition was not explicitly imposed to the calculation model but deviations from this principle were controlled and found to be negligible for the transport. The scheme that has been worked out allows the computation of changes in the concentration profiles and the electrical variables (current, true electrode potential, resistance and ohmic drop) as functions of time and support ratio, c supp. el / c substrate. The results obtained demonstrate that the true potential of the working electrode in a solution with deficit of supporting electrolyte differs significantly from the applied potential and changes in time. This makes chronoamperometry resemble voltammetry. The standard heterogeneous rate constant of the electron transfer strongly influences the way the current changes in time and therefore can be determined from chronoamperometric experiments.